The unexpected discovery of two fortification trenches surrounding a buried Native American village in North Dakota is providing new insights into the history of the Great Plains. The find suggests that the settlement was much larger and 300 years older than thought and includes hints of defensive tactics previously unknown in early warfare on the Great Plains.

The Double Ditch Indian Village State Historic Site, north of Bismarck, was long believed to be an 18th century village of the Mandans. These gardeners and buffalo hunters lived in large settlements along the Missouri River. Two ditches give the site its name. With wooden palisades on their inside edges, the ditches were of a kind used to defend against invaders such as the Lakota. This summer, using ground-penetrating radar, electrical resistivity, and magnetometry, a team led by archaeologist Kenneth Kvamme of the University of Arkansas, Fayetteville, found another pair of trenches--each nearly 2 meters wide and 1 to 2.5 meters deep. These newfound ditches appear to enclose about 7.5 hectares, or roughly 24 football fields.

The village was already considered one of the largest Great Plains settlements. The new findings, which Kvamme presented 24 October at the Plains Anthropological Society meeting in Oklahoma City, suggest that Double Ditch was an even more major North American settlement. Kvamme conjectures that it was home to as many as 3000 people, half again as many as previously thought. And it was long-lived too: Potsherds excavated from these ditches and the make of nearby buildings suggest they date to the 15th century.

The most curious finding, Kvamme says, is a series of earth-covered mounds of refuse alongside the ditches. The mounds' steep sides facing the ditches suggest to Kvamme that they could have been used as ramparts--an innovation long considered absent in the Great Plains before the arrival of Europeans. The mounds must have taken a phenomenal amount of work, says Kvamme's wife and collaborator, archaeologist JoAnn Kvamme. "It seems like you have to be really worried about something on the outside to move this much earth."

"These unexpected findings have certainly significantly changed and enlarged our view of Mandan prehistory," says archaeologist W. Raymond Wood of the University of Missouri, Columbia. He says the rampart hypothesis is speculative but plausible: "The mounds found do conform nicely with the ditches and would have been very helpful as heights."